Principle is simple: when coverage is deep enough, any single-base read error will look like a rare k-mer but correct sequence will be supported by many reads to correct sequences will look like common k-mers. So, algorithm has 3 main steps:
1. K-mer graph buildup.
For each read in an active region, a map from k-mers to the number of times they have been seen is built.
2. Building correction map.
All "rare" k-mers that are sparse (by default, seen only once), get mapped to k-mers that are good (by default, seen at least 20 times but this is a CL argument), and that lie within a given Hamming distance (by default, =1). This map can be empty (i.e. k-mers can be uncorrectable).
3. Correction proposal
For each constituent k-mer of each read, if this k-mer is rare and maps to a good k-mer, get differing base positions in k-mer and add these to a list of corrections for each base in each read. Then, correct read at positions where correction proposal is unanimous and non-empty.
The algorithm defaults are chosen to be very stringent and conservative in the correction: we only try to correct singleton k-mers, we only look for good k-mers lying at Hamming distance = 1 from them, and we only correct a base in read if all correction proposals are congruent.
By default, algorithm is disabled but can be enabled in HaplotypeCaller via the -readErrorCorrect CL option. However, at this point it's about 3x-10x more expensive so it needs to be optimized if it's to be used.
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